This invention relates to ring laser gyroscopes in general and more particularly to an improved piezoelectric actuator for a ring laser gyro.
A ring laser gyro, as its name implies, is a gyroscope which utilizes a laser beam directed to travel in a closed path, e.g., a ring, to detect rotation about the axis of the path around which the laser beam is directed. Typical ring laser gyroscopes are disclosed in U.S. Pat. Nos. 3,373,650 and 3,467,472. The ring laser gyroscope must be capable of operating over a wide range of temperatures. As a result, the material of which the gyroscope is made suffers thermal expansion and contraction as the temperature changes. The laser beam within the ring laser gyroscope is directed in its path by means of mirrors, typically in a triangular path between three mirrors. The temperature change resulting in expansion or contraction, causes a change in the path length. This change in path length, if not corrected, can result in a drift, i.e., an output indicating a rotation when there actually is none, and also results in scale factor changes. Thus, it is common practice to make one mirror with a flexible annulus and mount this mirror on a piezoelectric actuator which is controlled such as to maintain the path length constant even though temperature changes makes the material expand or contract.
One type of actuator is disclosed in U.S. Pat. No. 3,581,227. In the device disclosed therein, the casing for the stack of piezoelectric discs comprises an extension of the mirror structure with a cover thereover.
Typically, these piezoelectric actuators are designed to control the path length of the laser to an integral number of laser wave lengths. It is usually desired that an actuator at least have the ability to change the flexible mirror five free spectral ranges, e.g., to change the ring laser gyroscope from one resonance to a fifth higher or lower resonance. For operation with visible red helium-neon laser wave-lengths, this means the mirror must be able to move at least EQU .DELTA.L=(5.times.0.6328.times.10.sup.-6)/.sqroot.3 meters.
The typical prior art arrangement for accomplishing this is illustrated in FIG. 1. The ring laser casing 11 is typically made up of a material such as Cervit. It contains channels 13 through which the laser beam 15 is directed. In order that the laser beam travel in a closed path, mirrors 17 are provided for reflecting the laser beam. One mirror 17 is made with a flexible annular area to permit changes in the path length. In order to control this path length, an actuator indicated generally as 19, has been commonly used. The actuator includes a housing 21 made of a metal such as Invar so as to match the thermal expansion coefficient of the gyroscope casing 11. Disposed within the casing 19 are a plurality of piezoelectric discs 23 shown in plan view on FIG. 2 and in cross sectional perspective view on FIG. 3. Piezoelectric discs are well known in the art and are such as to change their thickness in response to a voltage. Although the voltage is typically provided by an amplifier for purposes of simplicity, the discs are shown on FIG. 1 as being energized from a battery 25 through a switch 27. Modern piezoelectric materials are capable of changing their thickness .DELTA.L/L=200.times.10.sup.-6 at full applied voltage. Thus, using the equation given above, the length of the piezoelectric stack must be L=0.0091 meter or 0.36 inches. As illustrated, the discs commonly in use have a hole 29 through their center. The piezoelectric material is normally divided into a plurality of discs typically 0.02 inch thick in order to permit the practical use of transistorized amplifiers where voltages typically are chosen not to exceed .+-.400V. The terminals are attached by means of thin spacers of metal 31, one of which is shown in perspective view on FIG. 4. The piezoelectric discs have the polarity indicated on FIG. 3 and are placed back-to-back, i.e., the positive of side of one disc to the positive side of the adjoining disc, with a spacer 31 therebetween, in order that, when energized, all discs expand and contract together. The common positive and negative leads 33 and 35 are brought out of the housing 21 through feed-throughs 37. The discs 23 and spacers 31 are held together by means of a nut 39 and bolt 41. The head of the bolt 39 being attached by appropriate adhesive to the mirror 3.
The primary disadvantage of this prior art actuator is its complexity. Many parts and electrical connections are required. Furthermore, the stack of discs 23 does not have great stiffness due to its rather small diameter dictated by space constraints and its relatively large height L. Stiffness is important because the alignment stability required of the mirrors is such that tilt cannot exceed 1 arc sec, anything more causing a noticeable effect. Furthermore, because a metal such as Invar is used as the housing material, the weight is increased and furthermore, the thermal expansion of the housing and that of the stack of discs will not cancel each other. This is a problem because it causes additional thermal expansion problems and makes it more difficult to maintain a constant path length.
A further disadvantage of a device such as that shown on FIG. 1 is that these devices are quite difficult to control with respect to arc-over. In order to prevent arc-over, the actuator must be treated with a conformal coating. It is almost impossible to assure complete coating on the inside of a closed cramped space such as in the housing 21. Thus, there is always a risk that a small spot might be missed and arc-over will occur, particularly at low ambient pressures.
In some instances, there have also been attempts to use bimorphs in actuators. A bimorph acts similar to a bimetal but is made up of piezoelectric material with radially expanding discs. Typically, one expanding disc and one contracting disc is used. The main drawback of such an arrangement is extremely low stiffness. If used in ring laser gyros, the membrane of the flexible mirror such as the mirror 17, must be made thinner than otherwise. Normally, the mirror membrane is cut to 0.04 inch. Great thinness greatly increases the price and also increases the risk of leaks.
Thus, it becomes evident that there is a need for a simpler actuator which provides greater stiffness to maintain better alignment stability.